What would you like Renovate to be able to do?

The automergeStrategy configuration item introduced in #10627 should be implemented for the Bitbucket Server platform, allowing users to configure the explicit merge strategy used by Renovate when automerging pull requests

Did you already have any implementation ideas?

The optional MergeStrategy argument can be provided to the Bitbucket Server mergePr implementation. If provided, the Renovate-specific strings for automergeStrategy can be mapped to Bitbucket Server-specific strings when submitting the merge request to Bitbucket Server. This can be provided as the strategyId to the POST request to .../pull-requests/${prNo}/merge. Not providing this field (as is the case now) will result in using the repository-defined merge strategy.

If #10830 has been implemented, the arguments to mergePr may be different than what has been listed here. We should also remember to clean up the missing feature notice in lib/platform/bitbucket-server/index.md and update the description in docs/usage/configuration-options.md if necessary.